It has been found that carbon nanotubes and carbon nanowalls, which are also referred to as carbon nano material, have better field emission characteristic, compared to the conventional field emitter of tungsten filament. Currently, the use of carbon nano material as a cathode has been applied to the manufacture of carbon nanotube field emission element and carbon nanotube field emission display.
FIG. 1 shows a conventional fully-flattened reflective plane light source, which includes a rod-shaped cathode 10, on which carbon nanotube field emitters 11 are formed, and an anode 20, which is a sheet glass 21 having a layer of metal reflective coating 22 deposited thereon and a layer of fluorescent powder 23 applied on the metal reflective coating 22. Due to a high electric field between the cathode 10 and the anode 20, electrons are released from the cathode 10 to impact the fluorescent powder 23 applied on the anode 20, causing the fluorescent powder 23 to illuminate. The metal reflective coating 22 further increases the luminous intensity of light emitted by the fluorescent powder 23. Since this type of reflective plane light source has limited number of releasable field emission electrons, and unevenly distributed internal electric field, it is uneasy to control an overall brightness and evenness of the luminescence thereof.
FIG. 2 shows another conventional reflective plane light source, which includes a cathode 30 having a plurality of field emitters 31 having carbon nano material formed on a metal rod or a metal sheet, and a sheet glass 41 on which a plurality of recesses 42 are provided and a metal reflective coating 43 is formed in each of the recesses 42. The field emitters 31 are separately disposed in the recesses 43 on the sheet glass 41 to form an array. This conventional reflective plane light source illuminates in the same manner as the fully-flattened reflective plane light source of FIG. 1. The recesses 42 with the metal reflective coating 43 may effectively reflect the light source to increase the luminous intensity. However, the conventional reflective plane light source of FIG. 2 could not be effectively utilized because a part of reflected light from the illuminated fluorescent powder 44 is shielded by the cathode 30.